Sapropterin dihydrochloride, chemical name (6R)-2-amino-6-[(1R,2S)-1,2-dihydroxypropyl]-5,6,7,8-tetrahydro-4(1H)-pteridinone dihydrochloride, molecular formula C9H15N5O3.2HCl, and CAS registry number 69056-38-28, is a synthetic product of tetrahydrobiopterin (BH4) dihydrochloride. BH4 is a cofactor of Phenylalanine Hydroxylase (PAH). Tyrosine is acquired from Phenylalanine (Phe) through hydroxylation under the action of PAH which is low in activity or even inactive in PKU patients, while BH4 is able to activate PAH, promote normal oxidative metabolism of Phe in the bodies of the patients, and reduce the Phe levels in the bodies of some patients. On Dec. 16, 2007, the sapropterin dihydrochloride tablets produced by BioMarin Pharmaceutical Inc. in USA were approved by the Food and Drug Administration (FDA) for marketing for treatment of PKU. Because of the effective activity of sapropterin dihydrochloride, it is extremely necessary to select a route applicable to industrial production with high product purity.
At present, BH4 is mainly synthesized by the following methods reported in literatures:    1. Preparation using 4-hydroxy-2,5,6-triaminopyrimidine (ATP) and 5-deoxy-L-arabinose as raw materials, please see literature E. L. Patterson et al., J. Am. Chem. Soc. 78, 5868 (1956).    2. Preparation using TAP and 5-deoxy-L-arabinose phenylhydrazone as raw materials, please see literature Matsuura et al., Bull. Chem. Soc. Jpn., 48,3767 (1975);    3. Preparation by reaction of raw materials hydroxyl-protected TAP and 4-acetyl-2,3-epoxypentanal through oxidation of iodine and a dehydroxylation protecting group, please see literature Matsuura et al., Chemistry of Organic Synthesis, Ml/g. 46. No. 6, P 570 (1988).
These traditional methods for preparing BH4 have the following major disadvantages: raw materials are expensive, arabinose which can be hardly acquired is used as a carbon atom radical for asymmetric synthesis; there are multiple steps in reactions with low yield, and low product purity, 5-deoxy-L-arabinose is easily degraded in a reaction solution, and products of the synthesis routes above have low stereoselectivity. To sum up, the traditional synthesis methods are not applicable to mass industrial production. Therefore, a synthesis route, which is applicable to industrial production with high product purity, high yield and high stereoselectivity, needs to be searched urgently.